I have had pretty good luck with GB. Just saw a note that they will be collecting sales tax January 1st.
I have had a few folks not follow through with winning an auction and getting your fees back is a pain. But you do get more for your items on GB even with the 6% in fees.
Armslist use to be good, but now they want you to buy a membership to even respond to sellers so it is going down hill fast.
Also lots of scams on Armslist like NIB Sig 229 for $400 shipped to your FFL Only.
As for fees I have a credit card that I hardly ever use and that is the one I give GB so I can manage it easier vs using the card I use every day. Might be an option for you.
If someone on GB has NR for no rating I watch it a bit more, but if they have over 20 or 30 feedbacks usually you are good. I always spend a little more to make sure I have the proper insurance and try to use UPS for better tracking.